1. Field of the Invention
This invention relates generally to calibration of pressure sensors for a hydrogen storage system and, more particularly, to a method and system for calibrating pressure sensors used in a hydrogen storage system for a fuel cell vehicle which uses a pressure reading from a low-range pressure sensor outside the hydrogen storage system to calibrate a high-range pressure sensor inside the hydrogen storage system, to reduce the tolerance band at the low end of the high-range sensor's range.
2. Discussion of the Related Art
Fuel cell vehicles are expected to rapidly increase in popularity in the near future in the automotive marketplace. Fuel cell vehicles offer several desirable features, such as virtually pollution-free emissions, and avoiding vehicle usage of petroleum fuels. A key component of fuel cell vehicles is the hydrogen storage system, which stores the hydrogen used as a fuel by most fuel cell vehicles. Hydrogen storage systems typically consist of one or more interconnected pressure vessels for storing gaseous hydrogen, along with numerous valves, gauges, and fittings necessary for operation of the hydrogen storage system. Maximizing vehicle driving range, while fitting within packaging constraints and meeting regulatory requirements, is a key consideration in the design of hydrogen storage systems for fuel cell vehicles.
Fuel cells require a reliable source of hydrogen gas at a prescribed pressure. A pressure regulator is used to reduce the hydrogen gas from the high pressure at which it is stored in the vessels to the lower prescribed pressure required by the fuel cell. However, the pressure vessels used in hydrogen storage systems cannot tolerate rapid refilling from an excessively low hydrogen pressure, so the fuel cell in a vehicle must be shut down if the pressure in the hydrogen storage system reaches a minimum allowable pressure value. Unfortunately, pressure sensors capable of measuring the high hydrogen pressures typically seen in the vessels are not very accurate at low pressure ranges. Therefore, in current hydrogen storage systems, it cannot be precisely determined when the pressure in the hydrogen storage system has reached the minimum allowable pressure. As a result, a tolerance band must be applied to the pressure reading, and the fuel cell must be shut down when the pressure reading minus the tolerance value reaches the minimum allowable pressure.
There is a need for a calibration methodology for a pressure sensor in a hydrogen storage system which improves the accuracy of pressure readings at the low end of the sensor's range. Such a method would enable a more precise reading of hydrogen pressure when the storage vessels are nearly empty, thus eliminating the large tolerance band around the pressure reading, avoiding the premature shutdown of the fuel cell, and allowing significantly more hydrogen fuel to be used to power the vehicle. As a result, the vehicle will have a greater driving range between refueling stops, which is a real benefit to the driver.